The following relates to the nuclear power reactor arts, nuclear reaction control apparatus arts, control rod assembly arts, and related arts.
In known nuclear power plants, a nuclear reactor core comprises a fissile material having size and composition selected to support a desired nuclear fission chain reaction. To moderate the reaction, a neutron absorbing medium may be provided, such as light water (H2O) in the case of light water reactors, or heavy water (D2O) in the case of heavy water reactors. It is further known to control or stop the reaction by inserting “control rods” comprising a neutron-absorbing material into aligned passages within the reactor core. When inserted, the control rods absorb neutrons so as to slow or stop the chain reaction.
The control rods are operated by control rod drive mechanisms (CRDMs). In so-called “gray” control rods, the insertion of the control rods is continuously adjustable so as to provide continuously adjustable reaction rate control. In so-called “shutdown” control rods, the insertion is either fully in or fully out. During normal operation the shutdown rods are fully retracted from the reactor core; during a SCRAM, the shutdown rods are rapidly fully inserted so as to rapidly stop the chain reaction. Control rods can also be designed to perform both gray rod and shutdown rod functions. In some such dual function control rods, the control rod is configured to be detachable from the CRDM in the event of a SCRAM, such that the detached control rod falls into the reactor core under the influence of gravity. In some systems, such as naval systems, a hydraulic pressure or other positive force (other than gravity) is also provided to drive the detached control rod into the core.
To complete the control system, a control rod/CRDM coupling is provided. A known coupling includes a connecting rod having a lower end at which the control rod is secured. The upper portion of the connecting rod operatively connects with the CRDM. A known CRDM providing gray rod functionality comprises a motor driving a lead screw that is integral with or rigidly connected with the connecting rod, such that operation of the motor can drive the lead screw and the integral or rigidly connected connecting rod up or down in a continuous fashion. A known CRDM providing shutdown functionality is configured to actively hold the control rod in the lifted position (that is, lifted out of the reactor core); in a SCRAM, the active lifting force is removed and the control rod and the integral or connected connecting rod fall together toward the reactor core (with the control rod actually entering into the reactor core). A known CRDM providing dual gray/shutdown functionality includes a motor/lead screw arrangement, and the connection between the motor and the lead screw is designed to release the lead screw during SCRAM. For example, the motor may be connected with the lead screw via a separable ball nut that is actively clamped to the lead screw during normal (gray) operation, and separates in the event of a SCRAM so that the control rod, the connecting rod, and the lead screw SCRAM together (that is, fall together toward the reactor core).
Related application Ser. No. 12/722,662 titled “Control Rod Drive Mechanism For Nuclear Reactor” filed Mar. 12, 2010 and related application Ser. No. 12/722,696 titled “Control Rod Drive Mechanism For Nuclear Reactor” filed Mar. 12, 2010 are both incorporated herein by reference in their entireties. These applications disclose configurations in which the connection between the motor and the lead screw is not releasable, but rather a separate latch is provided between the lead screw and the connecting rod in order to effectuate SCRAM. In these alternative configurations the lead screw does not SCRAM, but rather only the unlatched connecting rod and control rod SCRAM together toward the reactor core while the lead screw remains engaged with the motor.
The CRDM is a complex device, and is typically driven electrically and/or hydraulically. In the case of shutdown or dual gray/shutdown rods, the control rod system including the CRDM may also be classified as a safety related component—this status imposes strict reliability requirements on at least the shutdown functionality of the CRDM.
To reduce cost and overall system complexity, it is known to couple a single CRDM with a plurality of control rods via an additional coupling element known as a “spider”. In such a case all the control rods coupled with a single CRDM unit move together. In practice a number of CRDM units are provided, each of which is coupled with a plurality of control rods, so as to provide some redundancy. The spider extends laterally away from the lower end of the connecting rod to provide a large “surface area” for attachment of multiple control rods.
Although it is desired for the spider to have a large effective area, the spider also passes through the control rod support assembly. The support assembly guides the control rods as they are moved into or out of the reactor, so as to prevent control rod bowing or lateral movement of any control rod in any direction other than the desired “up/down” direction. The support assembly should cam against each control rod over a perimeter portion (transverse to the SCRAM direction) large enough to prevent rod bowing or lateral movement.
Another limitation on the spider's effective area is that during a SCRAM the spider should not present a large hydraulic resistance that limits acceleration of the detached control rod/spider/connecting rod/(and, optionally, lead screw) assembly toward the reactor core during a SCRAM. Since the spider's “effective surface” for attachment of rods is oriented broadside to the SCRAM direction, this is a substantial concern.
In view of these considerations, a spider typically comprises metal tubes or arms extending outward from a central attachment point at which the spider attaches with the connecting rod. In some spiders, additional supporting cross-members may be provided between the radially extending tubes, but the use of such cross-members is limited by the desire to minimize the actual area oriented broadside to the SCRAM direction. The diameters (or more generally, sizes) of the metal tubes or arms comprising the spider are kept as low as practicable in order to minimize hydraulic resistance of the spider during SCRAM and to enable the control rod support structure to contact and cam against all control rods during raising or lowering of the control rods. The spider is thus a lightweight, “spidery” structure having large lateral openings between the tubes or arms to reduce the actual surface area oriented broadside to the SCRAM direction. For various reasons such as strength and robustness, low cost, manufacturability, and compatibility with the reactor vessel environment, both the connecting rod and the spider are usually stainless steel elements.